What is the NSObject isEqual: and hash default function?

Question

I have a database model class that is a NSObject. I have a set of these objects in a NSMutableArray. I use indexOfObject: to find a match. Problem is the model object's memory address changes. So I am overriding the hash method to return the model's row ID. This however does not fix it. I also have to override the isEqual: method to compare the value of the hash method.

What does the isEqual: method use to determine equality by default?

I'm assuming it uses the memory address. After reading the isEqual: documentation I thought it used the value from the hash method. Obviously, that is not the case as my attempt to override that value did not solve my initial problem.

Answer 1

I would assume that NSObject isEquals uses the == operator, and hash uses the memory address.

isEquals method should never uses hash as an absolute test for equality. It is guaranteed to have two objects having similar hashCode, if you search for enough objects (just create more than 2^32 different objects, and at least two of them will have the same hash).

In other words, hash requires the following spec: If two objects are equals, then their hash needs to be equal; however, if two objects' hash values are equals, they are not necessarily equal.

As a tip, you always should override isEquals and hashCode together.